Conventional gas turbines typically include several combustion chambers (also referred to as “cans”) arranged in a circle about the axial centerline of the turbine. The combustion cans are isolated from one another, except for the crossfire tube connections between adjacent cans. The crossfire tubes are essentially open tubular structures that serve to propagate hot gases and flame between adjacent cans during start up under the influence of a pressure differential between the respective cans. Typically, one or two of the cans incorporate an ignition device (e.g., a spark plug), while the other cans are lighted by the flame passing through the crossfire tubes from the adjoining lit can. In addition, the crossfire tubes may also pass flame from the lighted to the unlighted premixing regions of the combustion cans during transfer from a premixed mode to a lean-lean mode. In general, the specific function of the crossfire tubes, whether during ignition or re-light of the premixing zone, is simply to pass flame from adjoining combustion cans. This process generally occurs in a matter of seconds. At all other times in the gas turbine operation, the crossfire tubes perform no specific function.
In theory, once all of the combustion cans are lit, their pressures equalize and the flow of gas and flame through the crossfire tubes should stop. In practical gas turbine engines, however, differences in geometry, air flow, and fuel metering between adjacent combustion cans may promote continuous gas and flame flow through the crossfire tube. Although a small amount of flow through the crossfire tubes does not affect the operation of the gas turbine engine and aids in balancing the pressures and flows from the combustion cans, continuous cross-flow of hot gas can permanently damage the combustion can liner or crossfire tube due to heating of the metal to its melting point.
One known method for discouraging continuous gas flow in crossfire tubes employs vent holes through the crossfire tubes. Pressurized purge air (from the compressor) flows inward through the vent holes and both cools any gas flowing in the crossfire tubes and counteracts the pressure differential along the length thereof. The purge air flow will prevent crossfire gas flow below a given pressure differential. In addition, the air flowing through the vent holes tends to cool the crossfire tube walls to reduce the temperature thereof. Reference is made, for example, to U.S. Pat. Nos. 5,896,742 and 6,334,294.
U.S. Pat. No. 5,001,896 describes a crossfire tube assembly that incorporates an impingement sleeve within which a crossfire tube is centrally disposed. The sleeve includes an array of cooling holes that direct cooling air upon the crossfire tube. The space between the impingement sleeve and the crossfire tube forms a flow channel along which the impingement air flows in the axial direction before being directed into the interior of the combustion cans.
Conventional crossfire tubes designed to prevent continuous crossfire by injection of pressurized purge air into the tube cavity through vent holes are disadvantageous in that the purge air bypasses the head-end of the combustion cans and thus is not available for the premixing of air and fuel supplied to the combustion cans, resulting in decreased efficiencies and increased emissions. This disadvantageous aspect also applies to the impingement sleeve configuration of the U.S. Pat. No. 5,001,896 discussed above in that the impingement air is eventually vented directly into the combustion cans without mixing with fuel at the head-end.
The industry would thus benefit from a robust and effective system for cooling crossfire tubes that does not decrease the amount of combustion air available for premixing with fuel at the head end of the combustion cans.